1. Field of the Invention
The present invention relates to a printing apparatus and a conveyance control method. Particularly, the present invention relates to a printing apparatus and a conveyance control method which perform accurate conveyance control even when, e.g., the leading edge or trailing edge of a printing medium enters between or passes through conveyance rollers.
2. Description of the Related Art
Recent printing apparatuses such as printers use not only plain paper but also printing media such as photo special paper to print photo images in many occasions. In particular, an inkjet printer which uses smaller ink droplets for printing can obtain an image quality equal to or higher than a film photo.
Accordingly, conveyance of printing media is also required to be accurate. Conveyance rollers use precision rollers with, e.g., a grindstone coating on a metal shaft. A DC motor used to drive the conveyance rollers is controlled by a cord wheel and an encoder sensor provided coaxially, thereby simultaneously ensuring high accuracy and high-speed conveyance.
Only one pair of conveyance rollers does not suffice for accurate printing of an image up to the trailing edge of a printing medium. To implement, e.g., marginless print, some proposed arrangements have another pair of conveyance rollers downstream in the printing medium conveyance direction. The mechanical accuracy of the conveyance roller pair downstream in the conveyance direction is also raised to ensure the conveyance accuracy (Japanese Patent Publication Laid-Open No. 2002-225370).
Additionally, to meet the growing requirements for a higher printed image quality and a higher printing speed, the print width of a printhead increases, the number of passes of multipass printing decreases, and the printing medium conveyance length of each pass printing increases. To attain higher image quality, ink droplets to be used in printing become smaller. This also indicates that it is necessary to more accurately convey a printing medium.
In this case, conventionally, encoder sensor control is applied to the above-described precision rollers to maintain accurate conveyance. However, in a printer having another conveyance roller pair downstream in the conveyance direction of a printing medium to cope with, e.g., marginless printing, after the trailing edge of a printing medium passes through the upstream conveyance rollers, when the downstream conveyance rollers solely convey the printing medium, drive transmission via, e.g., an idler gear also gets involved. The conventional arrangement that performs conveyance control by using only the encoder sensor output of the upstream precision rollers can hardly ensure the conveyance accuracy. To ensure the accuracy, the number of use nozzles of the printhead must be restricted. This is a great obstacle in speeding up printing.
To solve this problem, an arrangement has been proposed in which a cord wheel is coaxially provided even on the downstream conveyance roller to perform conveyance control using another encoder sensor. That is, the roller positions are detected by a plurality of encoder sensors, thereby ensuring high accuracy in the entire conveyance mechanism.
In the above prior art, however, the conveyance accuracy is lower in an area where conveyance is performed by a conveyance roller other than the main conveyance roller (e.g., precision roller) compared to an area where the main conveyance roller performs conveyance. For this reason, the area where conveyance is performed by a conveyance roller other than the main conveyance roller is reduced as much as possible with respect to the area of a whole printing medium. Thus, in a case where a conveyance correction amount is to be acquired by printing an adjustment pattern as described in Japanese Patent Publication Laid-Open No. 2004-122362, if the number of times of conveyance operations of a printing medium is very small, and/or if plural times of conveyance operations is performed, the print width per cycle must be very narrow. It is consequently difficult to detect an optimum correction amount from the printed adjustment pattern.